1. Field of the Invention
The present invention relates to a corrugation forming apparatus and a corrugation forming method adapted to form a plurality of streaky recesses and projections (i.e., a corrugate shape); in particular, to a corrugation forming apparatus and a corrugation forming method adapted to form a corrugate shape on a thin metal plate, as well as to a metal separator for a fuel cell having a plurality of streaky recesses and projections formed by the corrugation forming method.
2. Description of the Related Art
Heretofore, development of a fuel cell which is compact and exhibits high power generation efficiency has been demanded. For example, a solid polymer fuel cell includes a plurality of stacked cell units each having a sandwich structure composed of a separator having fuel passages, a fuel electrode, a solid polymer membrane, an air electrode, and a separator having air passages. In such a solid polymer fuel cell, the following means can be employed in order to reduce the size of the cell itself and improve power generation efficiency. That is, an example means for reducing the size of the cell is thinning the two separators. Further, an example means for improving power generation efficiency is defining fuel and air passages at fine pitches so as to accelerate reaction between fuel and air.
In order to realize the above-mentioned means, there have been proposed an apparatus and method for manufacturing separators for solid polymer fuel cells, each having a flat portion at the periphery, and a plurality of streaky recesses and projections formed thereon, as shown in, for example, Japanese Patent Application Laid-Open (kokai) No. H2002-313354. The conventional manufacturing apparatus disclosed in this publication has paired upper and lower pre-forming draft rolls at a former stage and paired upper and lower final-forming draft rolls at a latter stage. The pre-forming draft rolls have recesses and projections for previously forming a material to have an iterative cross-sectional shape having successive convex portions and concave portions. The final-forming draft rolls have recesses and projections for finishing the pre-formed material to have a final, iterative cross-sectional shape having successive convex portions and concave portions. In the conventional manufacturing apparatus, when the paired pre-forming draft rolls and the paired final-forming rolls are rotated, the shapes of the recesses and the projections are imparted to the material, thereby manufacturing a separator having an iterative cross-sectional shape having successive convex portions and concave portions.
However, in the conventional manufacturing apparatus and method, since successive projections and recesses are formed in a central portion of a material which has a flat portion at the periphery thereof, in some cases considerable distortion is generated and accumulated in the central portion (in particular, at the boundary between the central portion and the flat portion), possibly leading to generation of ductile cracks. Such ductile cracks become likely to be generated in particular during formation of a fine corrugate shape. Therefore, forming a fine corrugate shape while avoiding ductile cracks requires measures, such as employment of a multi-stage process; i.e., a multi-stage pre-forming process, which may increase cost of production.